The present invention relates to an electric power supply system for a motor vehicle such as a passenger car and more particularly to a dual voltage power supply system profitably suited for use as a power supply system for a motor vehicle in which the output voltage of an electric generator is boosted to permit the current capacity of wiring required for interconnection of various onboard devices to be reduced.
In the industrial field of motor vehicles such as passenger cars and others, there arises a tendency that the number of electric loads is increased as a result of efforts made to improve the comfortableness and manipulability of the vehicle, involving a problem that the onboard electrical wiring becomes very complicated. To cope with this problem, approaches for reducing the amount of wiring by boosting the source voltage are proposed in U.S. Pat. No. 4,694,238 and an article entitled "Dual Voltage Electrical Systems For Future Autos" contained in Design News published on Oct. 6, 1986, pp. 140-142. According to the concept of these prior art techniques, electric loads such as motor-driven actuators and the like are supplied with a high voltage of 24 V (volts) or 48 V, while a low voltage of 12 V is supplied to electric bulbs or the like which can not be rated for a high voltage because of poor vibration-withstanding capability of the filaments which would be required.
However, in the article mentioned above, no proposal is made concerning a technique for converting a voltage of a high voltage system to that of a low voltage system.
One of the most important problems encountered in realizing a high-voltage power supply system for a motor vehicle can be seen in how to implement the power supply systems of different voltages at low costs. As the device for the voltage conversion, there are heretofore known a DC-DC converter, a switching regulator and others. They are, however, very expensive.